1. Field of the Invention
The present invention relates to the technical field of a grooving apparatus and a grooving method and more particularly to the technical field of a grooving apparatus and a grooving method for forming a groove which is to be formed in an optical disk so as to function as an information track of the optical disk or the like.
2. Description of the Related Art
Recently, the DVD-R (Digital Versatile Disc-Recordable) or DVD-RW (Digital Versatile Disc-Re-Recordable) or the like has been coming into general use as a high-recording-density and recordable optical disk. In each of these recordable optical disks, two walls (wall surfaces) of a groove, which is formed in the optical disk so as to function as an information track on which information is recorded, have varying shapes so as to record information such as address information indicative of a record location at which information is recorded on the optical disk. More specifically, changes in the shapes of the walls are that the walls are waved in the radial direction of the optical disk, and thus, a frequency is modulated on the waved walls in accordance with information to be recorded so that the information is carried on the information track.
Recently, the GBR (Groove Baseband Recording) method has been designed as a method for carrying necessary information on the information track by using varying shapes of the walls of the groove as described above. The GBR method is described in detail by, for example, S. Kobayashi et al. in “GBR (Groove Baseband Recording) for an optical disc ROM”, Tech. digest of ODS2000, and so forth.
However, the above-mentioned conventional GBR method has the following problem. The GBR method is configured to correspondingly irradiate the walls of the groove with two laser beams and control the positions irradiated with the two laser beams in accordance with a shape signal indicative of a change in the shape of each wall, and therefore, it is impossible in principle to form the walls, each of which exhibits a shape change whose cycle is shorter than a length which is obtained from a length of a track pitch by dividing a wavelength of each laser beam by a numerical aperture of a condenser objective lens.
More specifically, the increase in an information recording density of the above-mentioned GBR method requires the reduction in the cycle of the change in the shape of each wall of the groove, but the minimum value of the track pitch that can be formed as the information track cannot be under about 0.3 μm even through the use of the GBR method using the laser beams capable of realizing the highest recording density under the current circumstances.
To form the groove while carrying information on the walls of the groove by using the GBR method, it is desirable for improvement in signal-to-noise ratio of the information under reproduction that the walls be formed as nearly perpendicular to an information recording surface of the optical disk as possible.
However, the above-mentioned conventional GBR method using the laser beams has difficulty in forming the walls perpendicular to the information recording surface, so that the walls are formed at an angle to the information recording surface. This results from exposure characteristics of a photoresist which is exposed to the beams of laser light when the walls are being formed. Therefore, the GBR method has another problem: the walls each having a gentle slope make it difficult to obtain an excellent reproduced information signal having a high signal-to-noise ratio because the GBR method is basically adapted to detect a change in the position of each wall in the radial direction of the optical disk as described above.